Conventionally, improving visibility of a display image on a display unit has been demanded. However, a display unit may not necessarily be in an environment that allows a viewer to easily see an image displayed on the display unit. For example, the brightness of external light penetrating into a display unit may affect the visibility of the display image.
That is, when the display unit is exposed to direct sunlight, visibility of a display image is lowered by sunlight reflected by the display unit. Particularly, when illuminance of sunlight is larger than an amount of light that reaches the viewer from the display unit, light that reaches the viewer from the display is dominated by the reflected sunlight more than gradation of the display image itself. Accordingly, when the display unit is exposed to direct sunlight, the display image that is output by the display unit becomes a white blurry image for the viewer. In other words, when light with high illuminance is irradiated to the display unit, the image the viewer sees is an image with a contrast lower than the contrast of the display image that is output on the display unit. As described above, losing difference of the gradation in the entire image leads to deterioration of the visibility.
Japanese Laid-open Patent Publication No. 2006-285063 discusses an image display apparatus that applies a tone correction and a saturation correction according to illuminance around a display unit. The tone correction refers to a correction that allocates more tone width to an area with a lower pixel value and allocates less tone width to an area with a higher pixel value in an input image. Hereinafter, the area with a lower pixel value in the input image is called a shadow part while the area with a higher pixel value in the input image is called a highlight part. The pixel value indicates a brightness and luminance of each pixel. For example, a pixel value is represented by a numerical value from 1 to 256. Meanwhile, the saturation correction increases color saturation so that similar colors can be easily distinguished. In other words, the saturation correction improves purity of a color.